In manufacturing electronic devices, dopants or impurities are introduced into a substrate to alter one or more of the underlying substrate's property. In manufacturing memory devices, boron ions may be introduced into a silicon substrate. As boron ions and silicon atoms in the crystal lattice have different electrical property, introduction of sufficient amount of boron ions may alter the electrical property of the silicon substrate.
A solar cell, another silicon substrate based device, may also be manufactured by introducing ions or dopants into the silicon substrate. Referring to FIG. 1, there is shown a cross-sectional view of a conventional selective emitter solar cell 100. The selective emitter solar cell 100 may comprise a p-type base 112. Adjacent to the p-type base 112, there may be a lightly doped n-type emitter 114. The selective emitter solar cell 100 may also comprise a plurality of heavily doped n-type contacts 122. Further, the selective emitter solar cell 100 may comprise a plurality of front side contacts 124 and a backside contact 126. To minimize light incident on the selective emitter solar cell from reflecting away from the selective emitter solar cell 100, an anti-reflective coating 132 may be disposed on the front side of the selective emitter solar cell 100.
Referring to FIG. 2, there is a cross-sectional view of a conventional interdigitated back contact (IBC) solar cell 200. IBC solar cell 200 may comprise an n-type base 212, on which an n-type front surface field 222, SiO2 passivating layer 224, and anti-reflective coating 226 may be disposed. As illustrated in the figure, the front side of the IBC solar cell 200 may have random pyramid configuration to additionally prevent light being reflected from IBC solar cell 200. On the back side, there may be p-type diffused emitter 232, n-type back surface field 234, a back side passivating layer 236, and a plurality of p-type contact fingers 242 and n-type contact fingers 244 alternately disposed. As illustrated in the figure, each p-type contact fingers 242 is in contact with p-type diffused emitter 232 via contact holes 250 through the backside passivating layer 232. Meanwhile, each n-type contact fingers 244 is in contact with n-type back surface field 234 via the contact holes 250 through the backside passivating layer 232.
The lightly doped n-type emitter 114 and heavily doped contacts 122 in the elective emitter solar cell 100, and p-type diffused emitter 232 and n-type backside surface field 234 in IBC solar cell 200 may be formed by providing dopants into the bases 112 and 222 of each solar cell 100 and 200. In the past, dopants have been introduced via diffusion process. In the diffusion process, dopant containing glass or paste is disposed on the silicon substrate. Thereafter, the substrate is heated, and the dopants in the glass or past are diffused into the substrate via thermal diffusion.
Although the diffusion process may be cost effective, the process has many drawbacks. For example, it is desirable to perform selective doping to introduce dopants to only selected regions of the substrate. However, the diffusion process is difficult to control, and precise doping via diffusion may be difficult to achieve. Precise doping may be desirable in forming for both elective emitter or IBC solar cells 100 and 200 as imprecise doping may lead to, among others, recombination of dopants or non-uniformity. In addition, voids or air bubbles, or other contaminants may be introduced into the substrate along with the dopants during the diffusion process.
To address such drawbacks, doping via ion implantation process has been proposed. In the proposed process, the substrate is coated with photo-resist layer, and lithographic process is performed to expose portions of the substrate. Thereafter, the ion implantation is performed, and dopants are implanted into the exposed portions. The process, although achieves precise selective doping, is not inexpensive. Additional steps and time to coat, pattern, and remove the photo-resist, each of which adds costs to the manufacturing process, are required. The steps may be more complicated if the regions to be exposed are extremely small. Another process that was proposed is a process of ion implantation using hard mask. This process also is a costly process requiring additional process steps.
Any added cost in manufacturing the solar cell would decrease the solar cell's ability to generate low cost energy. Meanwhile, any reduced cost in manufacturing high-performance solar cells with high efficiency would have a positive impact on the implementation of solar cells worldwide. This will enable the wider availability and adoption of clean energy technology.
As such, a new technique is needed.